Wireless communications systems are becoming the preferred choices for the provision of transmission services for digital voice, video and data. Code division multiple access (CDMA) technology is one of the effective wireless access technologies for supporting variable and high data rate transmission services. As an example, wideband CDMA (WCDMA) is adopted in the standardization of the third generation partnership project (3GPP).
It is therefore envisaged that there is substantial business potential in the provision of Internet services through wireless communications. In the wireless Internet domain, most of the data is transferred from a base station (BS) to a mobile station (MS), thus making downlinks the dominant traffic link in the wireless Internet domain. This situation is also true of other data transmission applications, e.g., wireless multimedia communications. Therefore, it is important that the performance of downlinks in wireless communications systems, in particular CDMA-based systems, is improved.
In the downlinks of CDMA-based systems, all mobile stations within the same cell or sector use the same frequency bands and time slots but different spreading codes for data transmission. Spreading codes consist of two layers of codes, namely long scrambling and short channelization codes. The long scrambling codes are common codes assigned to all mobile stations within the same cell or sector. However, the mobile stations are allocated unique short channelization codes which are dedicated codes orthogonal to each other. To support multi-data rate transmissions in CDMA-based systems, two spreading methods may be used, namely the multi-code (MC) and orthogonal variable spreading factor (OVSF) methods.
Communication channels allocated to the mobile stations are referred to as downlink traffic channels (DTCHs), through which data symbols intended for the respective mobile stations are conveyed. In order to establish and maintain the connections between the base stations and mobile stations, common channels, namely a common pilot channel (CPICH) and a common control channel (CCPCH), are also allocated to each cell or sector for conveying the relevant information shared by all the mobile stations within the same cell or sector. The data symbols conveyed by DTCHs, CPICH and CCPCH are orthogonally spread using spreading codes into data-bearing spread signals, then synchronously multiplexed, and finally transmitted through the same physical medium as a transmitted signal. In multi-data rate transmissions, the orthogonality between the spreading codes for low- and high-data rate DTCHs and the CPICH and CCPCH is maintained.
There may be many tall obstacles such as buildings and hills situated between base stations and a mobile station, and a wireless channel is therefore well modeled as a wide-sense stationary uncorrelated scattering (WSSUS) channel. In a CDMA downlink transmission system, a transmitted signal arrives at a mobile station together with several time-delayed, amplitude-scaled variants of the transmitted signal which travel along multiple paths in the wireless channel. A CDMA receiver resolves the multipath wireless channel into several paths which convey these multipath signals known as rays, the time delays of which are multiples of a spread signal chip interval Tc, and each ray is subject to statistically independent Rayleigh fading. The resolved ray with time delay τ represents a group of multipath signals with time delays over an interval [τ−Tc/2, τ+Tc/2]. If there is only one resolved ray, a frequency non-selective fading channel is observed. However, if there is more than one resolved ray, the wireless channel is called a frequency selective fading channel.
If the wireless channel is frequency non-selective, data symbols may be recovered at the mobile station using a conventional despreader without any intra-cell interference. However, a wireless channel is practically frequency selective because of the large time dispersion or time difference between the multipath signals arriving at the mobile station.
A conventional CDMA receiver in a mobile station employs a Rake combiner to coherently combine the despread outputs from all resolved rays determined by a path searcher, thereby recovering the transmitted signal.
A Rake receiver provides for path diversity and captures all resolved rays. However, there are two kinds of interference associated with a Rake receiver for CDMA downlink transmission, namely inter-finger interference (IFI) and multiple access interference (MAI), both of which are due to the frequency selectivity of the wireless channel. The capacity of a CDMA system with Rake receivers is limited by IFI and MAI.
In order to improve the performance of CDMA downlink transmission, CDMA receivers that provide for suppression of IFI and MAI are needed. When the delay dispersion is large, a frequency selective fading channel may be transformed into a frequency non-selective fading channel through channel equalization. Therefore, equalization receiver is an effective CDMA receiver for recovering data symbols by restoring the orthogonality of the spreading codes, thus suppressing both IFI and MAI.
FIG. 1 shows a block diagram of a conventional equalization receiver 100 in a mobile station for CDMA downlink transmission. The conventional equalization receiver 100 includes a cell searcher 102, a code generator 104, a path searcher 106, a despreader 110, an equalizer 108, and a signal detector 112.
In the equalization receiver 100, the cell searcher 102 receives from a CDMA downlink system a transmitted signal and any corresponding rays using multiple antennas that may employ oversampling, and retrieves therefrom long scrambling codes relating to a cell or sector in which the equalization receiver 100 operates, and information relating to cell and frame synchronization. The code generator 104 using the long scrambling codes retrieved by the cell searcher 102, generates a combination of long scrambling and short channelization codes known as spreading codes relating to the common channels CPICH, CCPCH and the corresponding DTCH required by the equalizer 108. The path searcher 106 then provides the time delay parameters of several rays with largest received powers by using the data symbols intended for the CPICH, which is known to the equalization receiver 100, the long scrambling and short channelization codes, and the received signals.
The equalizer 108 is described in greater detail with reference to FIG. 2, where there are M physical channels obtained using multiple receiver antennas that may employ oversampling, each of which is defined as a subchannel that includes a linear finite impulse response (FIR) hj(n) with maximum delay relating to L number of taps. For example, M=4 physical channels may be achieved via four receiver antennas without over-sampling, or via two receiver antennas, each of which employs 2-times over-sampling.
In the equalizer 108, there are M linear FIR filters 202, gj(n), each corresponding to a physical channel with response hj(n). During operation, a received signal received via a physical channel j is passed to into a corresponding FIR filter 202 of length G thereby producing a filtering output zj(n). A signal combiner 204 then sums the output from each FIR filter 202, generating an equalization output signal z(n) for further processing by the despreader 110. Equalizer coefficients used in the equalizer 108 may be obtained, for example, by minimizing the difference, using the minimum mean-square-error (MMSE) method, between the equalization output z(n) and delayed variant of the received signal, x(n−u), where u is known as a reference timing. The path searcher 106 provides such a reference timing required by the equalizer 108 during operation.
The despreader 110 then despreads the output of the equalizer 108 using the spreading codes allocated to the mobile station. The signal detector 112 then recovers from the output of the despreader 110 data symbols intended for the mobile station.
In a conventional equalization receiver, the FIR filter length G is chosen to be greater than or equal to the subchannel length L, so that energy from all taps in the respective subchannel may be captured. The number of taps therefore is typically chosen to be equal to the subchannel length L. Taps are sampled outputs corresponding to different time delays from a continuous signal measured against units of time delays sampled at a sampling frequency, for example at chip rate sampling frequency, where the continuous signal represents the channel response of the wireless channel between the base station and mobile station. Only a single equalizer is required when G is chosen in this way, and the minimum total number of equalizer coefficients required for the operation of the equalizer in the conventional equalization receiver is ML, which is deemed very large for a wireless communications system.
Typically in a conventional equalization receiver with a large number of taps, noise is increased and convergence problems arise when the equalizer is implemented using adaptive algorithms. The least mean square (LMS) algorithm, a type of adaptive algorithm, is not suitable for the acquisition of equalizer coefficients because of the typically long convergence time relating to such an algorithm. Conversely, adaptive algorithms which involve a short convergence time, such as the recursive least square (RLS) algorithm, are considered too complex for applications with a large number of taps. If the adaptive algorithm diverges, or converges slowly, it is difficult to achieve the suppression of IFI and MAI, which is an important objective of an equalization receiver.
There is therefore clearly a need for CDMA downlink receivers that suppress IFI and MAI, employ a simple receiver structure, and apply a fast convergence algorithm for addressing the foregoing problems.
Specification Text:
In accordance with a first aspect of the invention, there is described hereinafter a code division multiple access downlink receiver for providing wireless communication via a wireless channel between a base station and a mobile station in which the receiver is implemented. The receiver comprises a plurality of subchannels whereof each conveys at least one signal received from the base station, and a cell searcher for receiving signals in the plurality of subchannels and retrieving therefrom a common code relating to a cell. The receiver also comprises a code generator for generating a set of common and dedicated codes relating to at least one communication channel using the output of the cell searcher, and a plurality of equalizers for receiving the code generator output and equalizing the received signals in the plurality of subchannels, wherein each of the plurality of equalizers includes a plurality of filters wherein each of the plurality of filters corresponds to each of the plurality of subchannels.
In accordance with a second aspect of the invention, there is described hereinafter in a code division multiple access downlink receiver, a method for providing wireless communication via a wireless channel between a base station and a mobile station in which the receiver is implemented. The method comprises the steps of providing a plurality of subchannels whereof each conveys at least one signal received from the base station, and receiving using a cell searcher signals in the plurality of subchannels and retrieving therefrom a common code relating to a cell. The method also comprises the steps of generating using a code generator a set of common and dedicated codes relating to at least one communication channel using the output of the cell searcher, and using a plurality of equalizers for receiving the code generator output and equalizing the received signals in the plurality of subchannels, wherein each of the plurality of equalizers includes a plurality of filters wherein each of the plurality of filters corresponds to each of the plurality of subchannels.